In the recent drive for higher integration of ICs, it is desired to form a finer size pattern. When resist patterns with a feature size of 0.2 μm or less are formed by lithography, chemically amplified resist compositions utilizing the catalysis of acid are most often used in order to attain a high sensitivity and resolution. Since materials used in chemically amplified resist compositions must have a certain level of transmittance to energy radiation for use in pattern exposure, a suitable one is selected from numerous materials in accordance with the wavelength of energy radiation.
As is well known in the art, chemically amplified resist compositions which are commercially implemented at the present are generally composed of two main functional components: a polymer having acidic functional groups to render the polymer soluble in alkaline developer wherein some or all acidic functional groups are protected with acid labile protective groups so that the polymer is insoluble in alkaline developer, and an acid generator capable of generating an acid upon exposure to high-energy radiation. However, in order to attain a high resolution, the activity of the acid generated by the acid generator must be controlled, and a basic compound must be added as an additional component.
Base resins comprising an aromatic skeleton are used as a main component in KrF excimer laser and EB lithography, since the aromatic skeleton has high etch resistance and a phenolic hydroxyl group serves as the adhesive group to the substrate to provide good physical properties. For the EUV on which development works are concentrated as the next generation light source, there is a strong possibility of a resin comprising an aromatic skeleton being used as the matrix material.
Of the polymers comprising an aromatic skeleton, a polymer comprising recurring units of 4-hydroxystyrene is most commonly used so far. This polymer has a phenolic hydroxyl group of weak acidity within the recurring unit while this functional group exhibits good adhesion to the substrate as well as solubility in alkaline developer. If the polymer is combined with protection of the phenolic hydroxyl group with an acid labile protective group, or with (meth)acrylic recurring units protected with an acid labile protective group, then a switch of solubility in alkaline developer can be triggered by an acid catalyst. Based on this concept, a number of polymers have been proposed. In Patent Document 1, for example, a rectangular pattern having a line width of 70 nm is formed.
On the ArF lithography using ArF excimer laser light of shorter wavelength than KrF excimer laser, development efforts were made in order to form a resist pattern of a finer size than in the KrF lithography. The eventual technology is the immersion lithography which interposes a liquid having a higher refractive index than air (e.g., water, ethylene glycol or glycerol) between the projection lens and the wafer and provides the projection lens with a numerical aperture (NA) of 1.0 or greater. Since the base resin in the resist composition used in the ArF immersion lithography must be highly transparent at the wavelength 193 nm, polymers having an alicyclic structure rather than aromatic compounds are employed as the base resin. Suitable polymers having an alicyclic structure include, for example, polyacrylic acid and derivatives thereof, norbornene-maleic anhydride alternating polymers, polynorbornene, ring-opening metathesis polymerization (ROMP) polymers, and hydrogenated ROMP polymers. These polymers are more or less effective in that resins themselves are increased in transparency (see Patent Document 2).
On the other hand, the photoacid generators have also been studied. As described in Patent Documents 1 and 2, acid generators capable of generating a low-molecular-weight sulfonic acid are generally used. Patent Document 3 discloses an acid generator capable of generating carboxylic acid having fluorinated hydrocarbon substituent. Patent Document 4 describes an acid generator capable of generating a polymer-bound sulfonic acid.